1. Field of the Invention
The present invention generally relates to a system for detecting the behavior of automobile wheels and, more particularly, to a wheel behavior detecting system applicable in any anti-lock brake system traction control system designed to suppress excessive slippage and/or spinning of automobile wheels.
2. Description of the Prior Art
It frequently occurs that, when the automobile accelerator pedal is depressed too quickly while the automobile wheels are on a slippery road surface, the automobile wheels undergo such excessive spinning that a reduction in steering comfort and a loss of energy results. Attempts have hitherto been made, such as disclosed in any one of the Japanese Patent Applications No. 62-3802, No.61-287107 and No. 61-258867, to provide a traction control system which is designed to detect the occurrence of the actual or incipient wheel spinning by the utilization of a control variable Ft, represented by the following formula, to suppress the driving forces regardless of the extent to which the accelerator pedal is depressed: EQU Ft=Vd-Vn+d(Vd-Vn)/dt
wherein Vd represents the speed of rotation of the driven wheel or, simply, driven wheel speed, and Vn represents the speed of rotation of the non-driven wheel or, simply non-driven wheel speed.
On the other hand, it also frequently occurs that, when an abrupt braking is effected while the automobile is running on the slippery road surface, the braked wheels tend to undergo such excessive slippage that a lost of steering comfort produces an extremely dangerous condition. Therefore, attempts have also been made to provide an anti-lock brake system wherein a control variable Fa, represented by the following formula, is utilized to detect the occurrence of the actual or incipient wheel slippage to suppress the braking forces regardless of the extent to which the brake pedal is depressed: EQU Fa=(Vv-Vw)+d(Vv-Vw)/dt
wherein Vv represents the vehicle running speed and Vw represents the wheel speed.
In any one of the above discussed prior art systems, since the control variable Ft or Fa contains a differential of the wheel speeds, the control variable Ft or Fa tends to abruptly change when the automobile running on a rough and/or gravel road is accompanied by a slight irregular rotation of the wheels. If such an abrupt change of the control variable exceeds a predetermined threshold value, the system may determine that the excessive spinning or slippage is taking place actually or incipiently and demand that control be effected. However, where the slight irregular rotation occurs in the wheels without actual or incipient spinning or slipping, no control is needed and the automobile can resume its normal condition. Accordingly, what appears to be the actual or incipient occurrence of excessive spinning or slippage detected frequently when the automobile is running on a rough and/or gravel road, is not a true indication of such occurrence, but a pseudo occurrence of spinning or slippage which should not have been detected.
In order to avoid the detection of the pseudo-spinning or pseudo-slippage, the U.S. Pat. No. 3,717,384, for example, discloses the system wherein the threshold value is increased. According to the system disclosed in the above mentioned U.S. patent, it has however been found that the detection of the true locked condition (with which the system cannot resume a normal condition) occurring when the automobile is running on a bad road, such as a rough and/or gravel road tends to be delayed and the control to reduce the braking force tends to be effected a moment after the actual occurrence of the locked condition. Therefore, with the system of the above mentioned U.S. patent, problems associated with the behavior of the wheels often occur that the stability of the automobile being driven is lowered, the automobile undergoes excessive pitching, the deceleration of the automobile tends to be abruptly decreased, and so on.